(a) Field of the Invention
This invention relates to a method for preparing Zinc Sulfide powder, particularly to a method for preparing a single crystalline Zinc Sulfide powder and to a Zinc Sulfide based fluorescent substance using a Zinc Sulfide produced by the same method as a source.
(b) Description of the Related Art
Recently, a research for synthetic of a ceramic powder and interests of their commercial applications has been increased. The factor has been recognized a reason why the conventional method that powder has prepared by milling a mineral has a critical point which can not be obtain a ceramic having superior properties.
The Powder synthesis method by liquid such as the coprecipitation method, the sol-gel method, and the hydrothermal synthesis method has been actively studied because the method complements the conventional deficiency, develops new potentially and enables to get high value added ceramic products.
Especially, the hydrothermal synthesis method has been lively studied and used commercially because the method includes controlling particle size and shape, one of merits of liquid-powder synthesis method and moreover it enables to grow single crystalline particle at the situation of much lower temperature than solid reaction and controlling particle size and shape.
A powder synthesis method by liquid such as hydrothermal synthesis method is build-up, which is a kind of growth method from minute core to particle, and therefore it facilitates the composition of minute particles, meanwhile it has difficulty in composing high crystalline particles. Many studies, to overcome these problems, has tried to grow the particles after limiting the sizes of the first particles and apply them to reaction at high temperature and pressure of the supercritical point of water and has used high concentration of acid or ammonium to facilitate the solubility.
A powder synthesis process by liquid type is generally conducted by the two steps of core formation and growth of crystal. These two steps are necessary to control the size of the particles. However, at the stage of core formation, the more the cores are, the smaller sizes of the cores become and therefore secondary seed formation occurs and it is hard to make even and large particles in case that when the crystal grows the supersaturation is high or energy barrier of growth to larger particles is lower than core formation. The supersaturation of reactant liquid should be controlled properly to make the particles big and even. It is very important to control proper solvent, density of solute, temperature and solubility and choose additives for controlling the form of the particles in order to compose synthesizing a ceramic powder because the supersaturation can be controlled by mainly the concentration of solute and the solubility of solution.
Meanwhile, Zinc Sulfide (ZnS) powder, which is multi-functioning ceramic powder applied to a pigment, an infrared apparatus and a water purifier apparatus, has been studied actively especially in the field of fluorescent substance and widely used for fluorescent material of TV cathode-ray-tube.
In many fields using Zinc Sulfide (ZnS) powder, the particle size of the powder and controlling of the form is very much needed but there are many difficulties in using of Zinc Sulfide (ZnS) because there is no other method than solid synthesis method at a high temperature having difficulty in controlling properties and there's no study result regarding the composition of Zinc Sulfide (ZnS) powder by various kind of a solution method such as a coprecipitation method, a hydrothermal synthesis method, an emulsion method.
In relation to the conventional technique regarding the composition of Zinc Sulfide powder by liquid type, the following has been reported: it is possible to get spherical Zinc Sulfide particles in case of aging by keeping it 60° C. after precipitating a Zinc Sulfide seed particle from water solution of zinc nitrate, nitric acid and thoiacetamide (Deborah Murphy Wilhelmy, Egon Matijevic, Preparation and Properties of Monodispersed Spherical-colloidal Particles of Zinc Sulfide, J.Chem.Soc., Faraday Trans. 1(80), 563-570 (1984)).
And, an example has been reported that the Zinc Sulfide has been precipitated in presence of a variable negative ion using a variable zinc salt and thioacetamide as sources. Herein the reaction temperature is established from 60 to 70 □, and when the Zinc Sulfide particle is synthesized in presence of each nitrate ion, sulfate ion, or acetate ion, the particle size of first particle is about 13 nm, and the particle size of second particle is multiple crystals (Mufit Akinc, Ahmet Celikkaya, Synthesis of Polycrystalline ZnS Particles, Ceramic Trans12, 137-146, (1990), Ahmet Celikkaya, Mufit Akinc, Preparation and Characterization of Colloidal ZnS Particles, Mat. Res. Soc. Symp. Proc., 155, 31-36 (1989)).
Also, there is an example that a Zinc Sulfide is synthesized by after solving zinc oxide with sulfuric acid, reacting with thioacetamide at a temperature of about 80 □. Wherein the analysis result of the produced powder according to reaction time shows that the early reaction produce a crystalline powder of 1 μm, and the powder is gradually changed to spherical particle having a particle size of about 2 to 5 μm by wrapping a fiber (Richard Williams, P. N. Yocom, F. S. Stofko, Preparation and Properties of Spherical Zinc Sulfide Particles, J. Colloid Interface Sci., 106, 388 (1985)).
Additionally a study has been reported that Zinc Sulfide is synthesized from an organometallics. Herein the Zinc Sulfide is synthesized in toluene by using diethylzinc and sulfuric hydrogen as a main source. The produced powder is a spherical Zinc Sulfide particle having a particle size of 0.1 um that a small particle having a particle size of about 5 nm is aggregated (Curtis E. Johnson, Deborah K. Hickey, Daniel C. Harris, Synthesis of metal sulfide powders from organometallics, SPIE/Infrared and Optical Transmitting Materials, 683 (1986)).
A study using hydrothermal synthesis method has been reported in addition to the example prepared by precipitation method using a low temperature. Herein Zinc Sulfide (ZnS) powder is synthesized by hydrothermal reaction at a temperature of 150 □ using zinc acetate and sodium sulfide (Na2S). It is reported that the synthetic powder obtained after reacting for about 10 hours is a very small sized particle which the average particle size is 6 nm (Qian Yitai, Su Yi, Xie Yi, Chen Qianwang, Chen Zuyao, Yang Y, Hydrothermal Preparation and Characterization of Nanocrystalline powder of Sphalerite, Mat. Res. Bull., 30(5), 601-605 (1995)).
Meanwhile, a study has been reported that Zinc Sulfide based thin film is prepared by using the reactant source which used in the research of hydrothermal reaction and 4 N nitric acid solution for controlling pH (Qianwang Chen, Y. T. Qian, Z. Y. Chen, L. Shi, X. G. Li, G. E. Zhou, Y. H. Zhang, Preparation of Zinc Sulfide thin films by the hydrothermal method, Thin Solid Films, 272, 1-3 (1996)).
On the other hand, Braun tube applied to TV and monitor has a high precision more and more, accordingly fluorescent substance having a small size is demanded for them, and the conventional Zinc Sulfide is prepared by using a mechanical milling for obtaining a fluorescent substance having 5 μm below of particle size.